TIME-RESOLVED CUSPIDAL STRUCTURE IN THE WAVE-FRONT OF SURFACE ACOUSTIC PULSES ON (111)GALLIUM-ARSENIDE

The first experimental observation of the cuspidal structure in the su rface acoustic wave (SAW) front propagating from a point source in a c rystal is reported. Nanosecond SAW pulses are generated on the (111) s urface of GaAs by focused nanosecond laser pulses and detected with a cw probe laser beam. Multiple SAW arrivals are observed in the cuspida l region near the [11(2) over bar] direction. A peak of the SAW amplit ude due to ''phonon focusing'' is observed in the cusp direction. The measured SAW wave forms are well described by a model calculating the surface response to a localized pulsed force.